Joint replacement surgery seeks to replace portions of a joint with prosthetic components so as to provide long-lasting function and pain-free mobility.
For example, in the case of a prosthetic total hip joint, the head of the femur is replaced with a prosthetic femoral stem component, and the socket of the acetabulum is replaced by a prosthetic acetabular cup component, whereby to provide a prosthetic total hip joint.
In the case of a prosthetic total knee joint, the top of the tibia is replaced by a prosthetic tibial component, and the bottom of the femur is replaced by a prosthetic femoral component, whereby to provide a prosthetic total knee joint.
The present invention is directed to orthopedic prostheses for restoring the knee joint and, in particular, to improved prosthetic tibial components.
There is a long and varied history in the use of different materials for joint replacement prostheses. Some early attempts, such as stainless steel hip prostheses, were found to be reasonably successful and are still in use today. Other attempts, such as acrylic femoral head replacements or Teflon “TMJ” replacements, were found to be unacceptable and have been abandoned.
Currently, combinations of materials are generally used to form joint replacement prostheses.
More particularly, in the case of a prosthetic total hip joint, the prosthetic femoral stem component typically comprises a metal, and the prosthetic acetabular cup component typically comprises a metal seat with a plastic liner.
In the case of a prosthetic total knee joint, the prosthetic tibial component typically comprises a metal base topped with a plastic bearing surface, and the prosthetic femoral component typically comprises a metal.
The present state of the art is currently dominated by the use of three different materials: titanium and its alloys, cobalt-based alloys and polyethylene plastics. The two metallic materials are generally used for structural constructs (e.g., constructs that must carry the loads transmitted through the joint), and polyethylene is generally used as a bearing material in the joints (e.g., to slide or rotate against an opposing metallic component).
Ceramic bearing couples have also been used in the art to some extent, but their use is relatively limited due to price and strength considerations.
The vast majority of structural implant constructs are currently made from either titanium alloys (e.g., Ti6Al4V) or cobalt-based alloys (e.g. CoCr alloys, including CoCrMo alloys). These materials have different advantages and disadvantages.
More particularly, titanium alloys generally exhibit relatively high general fatigue strength, relatively low stiffness compared to alternative materials, and excellent biocompatibility properties. Titanium alloys, however, also tend to suffer from notch sensitivity in fatigue, which significantly reduces the fatigue strength of the construct when the surface is notched, roughened or porous-coated. Titanium alloys are also prone to scratching and make relatively poor sliding couples with polyethylene.
CoCr alloys generally have relatively high fatigue strengths, are relatively notch insensitive, and are relatively tough and resistant to scratching, thus making them excellent candidates for forming sliding couples with polyethylene. However, CoCr alloys are also relatively stiff, which can cause load pattern problems when coupled with flexible human bones, and they are not as biocompatible as many other alloys due to their chrome, and in some cases nickel, content.
In the 1980's, titanium alloys were used in many applications to take advantage of their biocompatibility. However, the applications that included sliding surfaces, such as femoral heads for the hip and knee femoral components, tended to have significant-problems with wear debris and scratching, and many exhibited clinical failure.
From this experience, implants were developed that combined the two aforementioned materials (i.e., titanium and CoCr alloys) in advantageous ways.
One early product was a knee femoral component that had a sliding surface of CoCr and a bone ingrowth surface of titanium. This design took advantage of CoCr's excellent wear characteristics in sliding articulations with the tibial component's polyethylene bearing, while still providing excellent bone ingrowth at the bone/prosthesis junction.
The aforementioned two materials (i.e., titanium and CoCr alloys) have also been used on hip femoral stem components. More particularly, hip femoral stem components have been developed which comprise an inner core of CoCr covered with a coating of titanium for bone ingrowth. This layered construction is desirable because stems made entirely of titanium, with titanium ingrowth surfaces, are too weak, while stems that are made entirely of CoCr, with CoCr ingrowth surfaces, do not have adequate biocompatibility. The combination of these two materials in a single construct provides a stem that is strong enough and also has a good bone ingrowth surface.
Another attempt to improve the biocompatibility of the bone ingrowth surface has been to coat the surface with hydroxyapatite (HA). However, HA, while it yields excellent short term results, has problems with long term stability due to its pH sensitivity. More particularly, the pH of the body may fluctuate due to a variety of conditions such as nutrition and disease, and this can undermine the effectiveness of HA bone ingrowth surface.
Another attempt to increase the hardness of the articulating surface has been to coat the articulating surface with a ceramic such as titanium nitride. The main limitation to this approach is that loading and abrading tend to undermine the mechanical integrity of the union between the ceramic coating and the substrate, and this can lead to prosthesis failure.
As wear issues relating to the main articulating surfaces have been addressed and incidences of gross and catastrophic wear eliminated, it has been discovered that the locking interface between the polyethylene bearing construct and the metal base construct can also be a significant source of wear debris. More particularly, it has been discovered that sliding motions in the junction between the polyethylene bearing construct and the metal base construct produce particles of polyethylene that can migrate out of the joint and into the body. Small abrasive particles can also migrate into the interface between the polyethylene bearing construct and the metal base construct and scratch the metal base construct, particularly where the metal base construct is formed out of titanium. This issue of “backside wear” has been a significant issue for research and debate over the last five years or so.
Attempts to address this issue have, to date, been limited to polishing the titanium mating surface of the metal base construct, as disclosed in U.S. Pat. No. 5,310,408 and as practiced in the “Reflection Cup” product marketed by Smith + Nephew of Memphis, Tenn. However, polishing a titanium surface has not worked well in previous attempts in sliding couples (i.e., in the femoral head component of a prosthetic total hip and in the prosthetic femoral component of a prosthetic total knee), and it has had only limited success in reducing wear debris at the locking interface between the polyethylene bearing construct and the metal base construct. This is primarily due to the inherent material limitations of the titanium metal base construct in the polished locking mechanism configuration.
No existing metallic construct that assembles with a polyethylene bearing is made of two metals (i.e., is bimetallic).
No existing bimetallic constructs lock with polyethylene.